squeeking under truck

Hey
I own a 2000 Silverado 2500 Ls 6.0L and recently i started hearing a squeeking noise sounds like from under the cab it squeeks when on idle and when driving sometimes the truck is loud so i dont hear it driving usually but i know its there. It squeals when i rev it in place and while driving im confused. some more info its a 5 speed, its not a HD but the heavy duty chassis was added, k&n intake, lockers.